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I've been thinking about ways of speeding up disk I/O, and one of the bottlenecks I keep coming back to is the journal. There's an obvious benefit to using an SSD for the journal - over and above just write caching unless of course I just disable the journal with the write cache (after all devicemapper doesn't seem to support barriers). In order to get the benefits from using a BB write cache on the controller, then I'd need to disable journalling - but then the OS should try to fsck the system after an outage. Of course if the OS knows what's in the batter-backed memory then it could use it as the journal - but that means it must be exposed as a block device and only be under the control of the operating system.

However I've not been able to find a suitable low-cost device (no, write-levelling for Flash is not adequate for a journal, at least one which uses Smartmedia).

While there's no end of flash devices, disk/array controllers with BB write caches, so far I've not found anything which just gives me non-volatile memory addressable as a block storage device.

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This is exactly what the FusionIO devices do, correct? – ErikA Feb 6 at 22:47
@ErikA: Still uses flash - might be using a more effective wear-levelling algorithm however cost is still exorbitant - was thinking more of something like DDRdrive - but even this is 5 x price of a laptop with the same DRAM capacity. – symcbean Feb 7 at 9:12
(or 10X the price of BBRAM write cache) – symcbean Feb 7 at 9:14
It seems Gigbyte used to make exactly what I've described here - marketed as an iRAM drive - anandtech.com/show/1742 – symcbean Feb 8 at 13:31
And there's something called a hyperdrive 5 - hyperossystems.co.uk/07042003/hardware.htm – symcbean Feb 8 at 13:40
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SSD drives should not be used for frequent write operations. Why do you not simply put the journal onto a separate fault-proof RAID system (5 or 10) consisting of ordinary disks (at least 4 "small" ones)?

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No - flash drives are write limited, so not suitable for frequent writes - they are not the only type of non-volatile solid state storage. – symcbean Feb 29 at 9:35
Right. This is what I wrote. Use a separate RAID of classical harddisks. – Nils Feb 29 at 13:31
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